Scr catalyst for removal of nitrogen oxides

ABSTRACT

The present invention provides for catalysts for selective catalytic reduction of nitrogen oxides. The catalysts comprise metal oxide supporters, vanadium, an active material, and antimony, a promoter that acts as a catalyst for reduction of nitrogen oxides, and at the same time, can promote higher sulfur poisoning resistance and low temperature catalytic activity. The amount of antimony of the catalysts is preferably 0.5-7 wt %.

TECHNICAL FIELD

The present invention relates to catalysts for selective reduction ofnitrogen oxides, and more particularly to catalysts for removal ofnitrogen oxides that have enhancing effects on the reduction activity ofnitrogen oxides at low temperatures and on the sulfur poisoningresistance.

BACKGROUND ART

Nitrogen oxides (NO_(x)) are usually produced when fuels are combusted,and are exhausted from moving sources such as a motor vehicle and fixedsources such as a power plant or an incinerator. These nitrogencompounds are identified as the major causes of acid rain and smogformation. Since environmental protection regulations have becomestricter recently, more studies are being carried out, in response, inorder to reduce nitrogen compounds through catalysts.

As a method of removing nitrogen compounds that were emitted from fixedsources, selective catalytic reduction (SCR) device that uses vanadiumoxides (V₂O₅) as active materials impregnated on a titanium oxidesupporters have been generally used. Ammonia has been known as a mostsuitable reduction agent for the system.

However, for the titanium-type SCR catalysts that use ammonia as areductant, a catalyst that operate under 300° C. is frequently requiredaccording to the working condition. Additionally, in case of a flue gaswhich contains sulfur oxides that easily poison the catalysts at lowtemperatures, catalysts that could with this problem also need to bedeveloped.

For the V₂O₅/TiO₂ SCR catalyst, high catalytic de NOx activity isexhibited at 300 ° C. or higher. Therefore, it is necessary to develop acatalyst which shows high activity at a lower reaction temperature.Generally, when titanium oxide (TiO₂) supporters and vanadium (V) areused as active catalytic materials, additional amount of vanadium isadded to increase the catalytic activity at 300° C. or lower. However,when the amount of vanadium is increased, the oxidation of sulfurdioxide(SO₂) that are contained in the exhaust gas to sulfur trioxide(SO₃) is induced, which then react with slipped ammonia. As a result,ammonium bisulfate, NH₄HSO₄ which is a solid salt, is formed.

The produced ammonium bisulfate salts are imbedded into the surfaces ofthe catalysts, thereby interfering with the reduction reaction. As aresult, as the amount of unreacted ammonia increases, formation ofsulfur trioxides (SO₃) is promoted, thereby accelerating the sulfurpoisoning, which eventually shorten the life of the catalysts.

Therefore, catalysts that can improve catalytic activity at lowtemperatures without promoting the oxidation of sulfur dioxides havebeen developed. In general, in order to enhance low temperature activityand sulfur poisoning resistance, tungsten has been added tovanadium/titania catalysts as a promoter. For example, when tungstenoxides were added, sulfur poisoning resistance at low temperatures couldbe increased.

However, since the amount of tungsten oxides used is high, approximatelybetween 5 wt. % and 10 wt. %, the increase in the price of catalysts isunavoidable.

Moreover, most of the conventional catalysts for removal of nitrogenoxides with less sulfur poisoning have been developed such that asupporter is impregnated with special active materials.

A conventional art uses a TiO₂ supporter impregnated with vanadiumsulfate (VSO₄), vanadyl sulfate (VO SO₄) and the like, and is reacted atthe range of temperatures at 300-520° C. However, the problem of thepreviously-explained sulfur poisoning also arises in this case due tothe usage of vanadium.

According to another conventional art, TiO₂ supporter impregnated withactive materials such as V₂O₅, MoO₃, WO₃, Fe₂O₃, CuSO₄, VOSO₄, SnO₂,Mn₂O₃, Mn₃O₄ are used. However, not only the problem of the sulfurpoisoning from vanadium oxides still exists, but also, thepreviously-mentioned high cost problem due to the usage of tungstenoxides are accompanied.

DISCLOSURE OF INVENTION

The present invention provides for catalysts for the reduction ofnitrogen oxides that are impregnated in to supporters and containvanadium as an active material and antimony as a promoter that promotereduction of nitrogen oxides at low temperatures and increase sulfurpoisoning resistance.

Another embodiment of the present invention provides for the transitionmetal oxides supporters, titanium oxides, silicate, zirconia, aluminaand the mixture thereof, where vanadium and antimony can be impregnated.

Another embodiment of the present invention provides that the amount ofsaid vanadium impregnated is 1-3 wt. %.

Another embodiment of the present invention provides that the amount ofsaid antimony impregnated is 0.5-7 wt. %.

As mentioned above in the conventional arts, nitrogen oxides can bereduced to harmless nitrogen and water by using a reductant. Catalystsfor the reduction of nitrogen oxides are used and each of thesecatalysts comprise a supporter, an active material and a promoter whichreduces sulfur poisoning and enhancing low temperature catalyticactivity.

For the supporter, titanium oxides, silicate, zirconia, alumina and themixture thereof can be used. Preferably, titania (TiO₂) is used.

Moreover, active and promoting materials comprise materials such asvanadium and antimony, respectively. The vanadium includes a compounds(solution) that contains vanadium oxides, and the antimony (Sb) includescompounds(solution) that contains antimony oxides, antimony chlorides(SbCl₃) and the like. Among the impregnated active and promotingmaterials, vanadium oxide is used as a main catalyst and the antimonyoxide is used as an auxiliary catalyst.

The present invention uses titanium oxide (TiO₂) as a supporter tocombine the vanadium (V) and antimony (Sb) to prepare catalysts for thereduction of nitrogen oxides. When preparing the catalysts, impregnationmethod, which uses the TiO₂ and precursors containing vanadium andantimony, or other conventional catalyst synthesis methods such as solgel method can be used.

According to the present invention, antimony is added to promote thereactivity at low temperatures and the sulfur poisoning resistance.Preferably, 0.5-6 wt. % of antimony is added. By the addition ofantimony as a promoter, the addition amount of vanadium can be reduced,and thus, the sulfur poisoning resistance can be reduced. Preferably,1-3 wt. % of vanadium is added.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the NO conversions of Example 1 and Reference1 at different temperatures.

FIG. 2 is a graph showing the sulfur poisoning resistance of Example 1and Reference 1 when ammonia was used as a reductant at 240° C.

FIG. 3 is a graph showing the sulfur poisoning resistance of Example 1and Reference 2 at 230° C.

FIG. 4 is a graph showing the NO conversions of Examples 1 to 7 andReference 1 at different temperatures.

FIG. 5 is a graph comparing the sulfur poisoning resistance of Examples1 to 7 with Reference 1.

MODE FOR THE INVENTION

The present invention will be further illustrated by the followingexamples in order to provide a better understanding of the invention.However, the present invention is not limited to the examples, andparticularly, the substances that compose each layer can be othersubstances that are within the technical effect of the presentinvention.

FIG. 1 shows NO conversion without the presence of antimony according toReference 1 (standard 1) and one with antimony at different temperaturesaccording to Example 1 (type 1) of the present invention.

Reference 1 uses titanium oxide (TiO₂) carrier, without antimony addedand impregnated with 2 wt. % of vanadium as an active material. Example1 uses titanium oxide (TiO₂) carrier which is impregnated with 2 wt. %of vanadium as an active material and 2 wt. % of antimony oxide as aminor catalyst. The amounts of nitrogen oxides and ammonia used are each800 ppm, the amount of water is 6%, and the amount of oxygen is 3%.

FIG. 2 shows sulfur poisoning resistances of Example 1 (type 1) withantimony added and Reference 1 (standard 1) without antimony added whenammonia was used as a reductant at 240° C. The same results wereobserved for Reference 1 and Example 1 as is shown in FIG. 1, and theamount of nitrogen oxides and ammonia used were each 800 ppm. Moreover,the amount of water and oxygen used were 6% and 3%, respectively. InFIG. 2, Reference 1 (NH₃) line and Example 1 (NH₃) line each representthe amount of unreacted ammonia, and Reference 1 (SO₂) line and Example1 (SO₂) line each represent the amount of sulfur dioxides.

As shown in FIG. 2, in case of a high NO removal rate as in Example 1(type 1), since most of the ammonia provided is exhausted during the NOremoval process, the amount of unreacted ammonia can be decreased, andthe amount of emitted sulfur dioxide of is nearly similar to the amountof the provided sulfur dioxide of 500 ppm, it can be inferred thatalmost no oxidation of sulfur dioxide occurred.

However, it is shown in Reference 1 that the amount of unreacted ammoniais increased after about 10 hours, and the amount of sulfur dioxide isdecreased due to oxidation. The reduction of the NO conversions afterabout 10 hours, also called deactivation, was clearly indicated.

Example 1 (type 1), which added antimony as a minor catalyst, showedchanges of the amounts of unreacted ammonia and sulfur dioxide after 16hours. Thus, not until after 16 hours, it could be determined that thesulfur poisoning occurred. Therefore, as shown in FIG. 2, when antimonywas added as a promoting catalyst, the sulfur poisoning resistance wasincreased.

FIG. 3 compares the sulfur poisoning resistance of Example 1 with thatof another Reference 2 (standard 2) using another catalyst at 230° C.Example 1 (type 1) is under the same condition as mentioned above,reference 2 represents a common catalyst that is impregnated with 1 wt %of vanadium to a titanium oxide carrier and 10 wt % of tungsten as apromoting catalyst.

The injected nitrogen oxides and ammonia amounts are each 200 ppm, andthe amount of sulfur dioxide is also 200 ppm. Moreover, the amounts ofwater and oxygen are 12.3% and 3%, respectively.

As shown in FIG. 3, in case of a high removal rate according to Example1, the increase in the amount of unreacted ammonia at different timeperiods was smaller than Reference 2 (standard 2), and the decreaseamount of sulfur dioxide compared to Reference 2 was also smaller.Accordingly, Example 1 was shown to exhibit a remarkably higher sulfurpoisoning resistance than the conventional catalyst of Reference 2.

FIG. 4 and FIG. 5 represent sulfur poisoning resistances and the NOconversion of Reference 1 (standard 1) and Examples 1 to 7 (types 1 to7).

Example 1 (type 1) and Reference 1 (standard 1) are same as explainedabove.

Example 2 (type 2) represents catalysts that were prepared byimpregnating a titanium oxide (TiO₂) carrier with 2 wt. % of vanadiumand 1 wt. % of antimony. Example 3 shows catalysts that were prepared byimpregnating a titanium oxide (TiO₂) carrier with 2 wt. % of vanadiumand 0.5 wt. % of antimony. Example 4 shows catalysts that were preparedby impregnating a titanium oxide (TiO₂) carrier with 2 wt. % of vanadiumand 3 wt. % of antimony. Example 5 (type 5) shows catalysts that wereprepared by impregnating a titanium oxide (TiO₂) carrier with 2 wt. % ofvanadium and 5 wt. % of antimony. Example 6 (type 6) shows catalyststhat were prepared by impregnating a titanium oxide (TiO₂) carrier with2 wt. % of vanadium and 7 wt. % of antimony. Example 7 (type 7) showscatalysts that were prepared by impregnating a titanium oxide (TiO₂)carrier with 2 wt. % of vanadium and 10 wt. % of antimony. In FIG. 4 andFIG. 5, the amount of nitrogen oxides and ammonia added are each 800ppm, 500 ppm for sulfur dioxide, and 6% and 3% for water and oxygen,respectively.

First, as shown in FIG. 4, the removal activity at low temperaturesaccording to Examples 1 to 6 (types 1 to 6), except for Example 7 (type7), was shown to be higher than that of Reference 1. Therefore, it wasshown that the amount range of antimony that increases the removalactivity at low temperature is 0.5˜7 wt %. There can be a deviation ofthe amount range of antimony due to the standard of error.

Moreover, the amount of vanadium added is preferably 2 wt %, howeverconsidering the conventional of error of the process, it is preferred toadd a range of 1˜3 wt %. According to FIG. 5, other than in Example 7(type 7), Examples 1 to 6 (types 1 to 6) showed an increase in theamount of unreacted ammonia and a decrease in the amount of sulfurdioxide with time compared to Reference 1. Accordingly, it can be shownthat Examples 1 to 6 all have an increased sulfur poisoning resistancecompared to Reference 1. Therefore, the amount range of antimony thatincreases the sulfur poisoning resistance is 0.5˜7 wt %. There can be adeviation of the amount range of antimony due to a conventional of errorof the process. Additionally, although a vanadium addition amount ispreferably 2 wt %, the range of 1˜3 wt % considering the standard oferror.

1. A ammonia SCR catalyst for reduction of nitrogen oxides comprising: acarrier; and vanadium oxide as an active material on the supporter; andantimony as a promoter that reduces sulfur poisoning and enhances lowtemperature catalytic activity.
 2. The catalyst of claim 1, wherein thesupporter is at least one from the group consisted of titanium oxide,silicate, zirconia, alumina and the mixture thereof.
 3. The catalyst ofclaim 1, wherein 1˜3 wt. % of the vanadium is used.
 4. The catalyst ofclaim 1, wherein 0.5˜7 wt. % of the antimony is used.